There are number of waveguide structures that can be used to realize an optical waveguide in silicon, such as ridge and channel waveguides. In such structures, light is typically guided in a high refractive index material (typically referred to as the waveguide core) that is surrounded by a lower index material (typically referred to as the waveguide cladding).
A channel waveguide is usually formed by depositing a high refractive index core material on a low refractive index bottom cladding material. Excess of the high refractive index material to either side of the channel is removed down to the underlying oxide using standard lithography processing (i.e., mask and etch). Once the channel is formed, a low refractive index upper cladding is deposited around the channel. The mismatch in refractive index between the core and the cladding effectively operates to contain radiation within the channel of the waveguide.
A ridge waveguide is a variation on the channel waveguide, wherein the high refractive index core material is only partially etched back to the underlying oxide, leaving a so-called slab to either side of the ridge. This lowers the in-plane refractive index contrast, which generally decreases scattering loss.
In some applications, both ridge and channel type waveguide structures are used. In such cases, the slab of the ridge waveguide is etched after the channel waveguides are fully formed. In particular, a small window in resist is opened to etch the small slab area, since other finished waveguide structures have to be protected from the etching process associated with forming the slab. This subsequent slab etch processing generally leads to large etching non-uniformities due to uneven plasma loading and increased sidewall roughness of waveguides. Moreover, the process typically does not yield repeatable results.
What is needed, therefore, are techniques that facilitate the fabrication of silicon-based circuitry including channel and ridge waveguide structures. In a more general sense, there is a need for more efficient techniques for fabricating semiconductors including waveguide structures.